1. Field of the Invention
The invention relates to a method for operating a steam cycle process with a lubricated expander based on the positive displacement principle, and to an apparatus for operating a steam cycle process.
2. Background of the Invention
Steam cycle processes with expanders are known for example from DE 10 2007 020 086 D3. The expander may for example be in the form of a piston expander, vane expander, rotary piston expander, swashplate expander, oblique-disk expander, roots expander or screw expander. In the positive displacement principle, the fresh vapor conducted out of the vapor generator is conducted into the working chamber of the expander. The fresh vapor conducted into the working chamber is expanded in the working stroke, with an output of work, owing to a volume-expanding movement of components. The expanded vapor, when it reaches its greatest volume, is conducted out of an outlet opening into a vapor discharge line. As vapor, use may be made not only of water vapor but also, as is known, of other inorganic and organic volatile substances, for example ammonia, alkanes, fluorinated hydrocarbons, siloxanes and refrigerants in general.
Many expanders must be lubricated with a dedicated lubricant, wherein the working medium and lubricant come into contact. In the remainder of the circuit, the working medium is completely liquefied in the condenser, raised to a higher pressure in the pump, and at least partially evaporated in the vapor generator.
A major problem in the cycle process is the selection of the lubricant. Since most lubricants are heat-sensitive, as complete as possible a separation of the lubricant from the working medium upstream of the evaporator is one option for making it possible to use heat-sensitive lubricants.
To be able to realize fuel savings, in particular in the case of mobile internal combustion engines, such as for example motor vehicle internal combustion engines, priority is presently given to two technical solutions. Different hybrid concepts are expedient for city and distribution transport owing to the braking and acceleration processes which arise there. Heat recovery systems are also known, which utilize the waste heat of the internal combustion engine to provide additional drive energy. Such systems for waste heat utilization are expedient, in the case of mobile internal combustion engines, in particular for vehicles used for long-distance transport.
In waste-heat utilization systems of this type, the waste heat generated in the region of the internal combustion engine and/or in the exhaust line is at least partially transmitted to a secondary heat circuit. In the secondary heat circuit, a working medium is circulated and in the process is usually at least partially evaporated in an evaporator. The vapor is expanded in an expansion unit, for example in a piston expander, and is finally liquefied again in a condenser. The condensed working medium is thereafter raised to the evaporation pressure by means of a pump unit, and the circuit is thus closed. The mechanical work generated by the expansion unit is supplied as additional work to the drive system, such as to a vehicle drive system.
DE 10 2006 043 139 A1 discloses a heat recovery system for an internal combustion engine. In the described system, additional drive energy is provided to the vehicle from the waste heat of the internal combustion engine and/or of the exhaust system. After the expansion of the vaporous working medium in the expander, the working medium of the secondary heat circuit is conveyed into a condenser in which it is liquefied with an output of heat, such that the corresponding steam cycle process is closed.
The use of expanders for the utilization of waste heat of internal combustion engines requires a complex design. To be able to meet all demands with regard to weight, costs, durability and necessary servicing, components which abrade against one another, for example piston-cylinder pairings, plain bearings, slides etc., are lubricated with oil. As a result, there is contact between the working medium and the lubricant or lubricated surfaces. This gives rise to the problem of mixture of the two working media, which may be transported onward in the circuit in the direction of the pump and evaporator together; this may have many adverse side effects.